Crown and collar rot, caused by at least four Phytophthora spp. (Jeffers et al, 1982, Phytopathology, 72:533-538) is a widespread and economically serious problem of apples throughout the Northeast U.S. In New York State, this disease appears to be the most common biological cause of premature tree decline and death, and in Pennsylvania, many growers have abandoned the horticulturally-desirable MM 106 rootstock because of high crown rot incidence or its perceived threat. Because no single approach to the control of Phytophthora crown rot has proved reliable, growers have been advised to adopt an integrated or additive disease management strategy, utilizing a combination of site selection, site modification, rootstock selection, and chemical treatments where appropriate (Wilcox, In: 1987 New York State Pesticide Recommendations. Cornell University, Ithaca, N.Y. 533 pp). However, the possibility of introducing a biological control agent as an additional component of such a program has heretofore been largely ignored.
Species of Trichoderma and Gliocladium have been shown to provide varying levels of biological control of a number of important soil-borne plant pathogens, including Sclerotium cepivorum (Abd-el Moity et al, 1982, Phytopathology, 72:396-400; Abd-el Moity et al, 1981,, Phytopath. Z., 100:29-35), Armillaria mellea (Ohr et al, 1973, Phytopathology, 63:965-973), Rhizoctonia solani (Chet et al, 1981, Phytopathology, 71:286-290; Elad et al, 1982, Plant and Soil, 66:279-281; Ruppel et al, 1983, Crop Protect., 2:399-408), Verticillium dahliae (Jordan et al, 1978, Ann. Appl. Biol., 89:139-141; Marios et al, 1982, Plant Dis., 66:1166-1168) and Pythium spp. (Chet et al, 1981, Microb. Ecol., 7:29-38; Hadar et al, 1984, Phvtopathology, 74:106-110; Harman et al, 1983, Seed Sci. & Technol., 11:893-906; Sivan et al, 1984, Phytopathology, 74:498-501). While there have been many recent advances in the use of Trichoderma spp. as biological control agents (Papavizas, 1985, Ann. Rev. Phytopathol., 23:23-54), to date there have been no concerted efforts made to use these fungi to control diseases caused by soil-borne Phytophthora spp., the economic importance of such diseases and the close relationship of the Phytophthora and Pythium genera notwithstanding.
Despite the lack of direct evidence, there exists correlative evidence that certain Trichoderma spp. may be involved in the biological control of several diseases caused by Phytophthora spp., e.g., T. viride versus heart rot of pineapple caused by P parasitica (Papazivas, 1985, suora). More compelling correlative evidence is supplied by the well-documented ability of composted hardwood bark (CHB) to provide control of Phytophthora disease of woody plants when incorporated into their rhizospheres (Hoitinak et al, 1986, Ann. Rev. Phytopathol.,24:93-114), including control of crown rot of apple under field conditions (Ellis et al, 1986, Plant Dis., 70:24-26), and the related documentation that the addition of CHB to a container potting mix resulted in a 100 to 100,000 fold increase in the population levels of T. harzianum in this rooting medium (Nelson et al, 1983, Phytopathology, 73:1457-1462).
It has been recently pointed out that the potential of Trichoderma spp. for use as biological control agents has been studied primarily as an end in itself rather than as a synergistic or additive component in a broader integrated pest management system (Papavizas, 1985, supra). Nevertheless, there are several studies in which such an approach has been attempted successfully, e.g., methyl bromide plus Trichoderma for the control of Armillaria mellea (Ohr, 1973, supra), and T. harzianum plus PCNB for the control of root rot and damping off of pea caused by Pythium ultimum (Kraft et al, 1983, Plant Dis., 67:1234-1237).